As the important UV radiation absorption wavelength region for a sunscreen cosmetic composition, it includes a UV-A region (320 to 400 nm) and a UV-B region (290 to 320 nm). Heretofore, it has been considered that the UV radiation in a UV-A region could cause darkening or tanning of the skin but would not cause sunburn, thereby to promote skin aging like the UV radiation in a UV-B region. Recently, however, it has become known that the UV-B radiation could stay relatively in the surface part of the skin while the UV-A radiation could reach the deep part of the skin to be therefore a cause of inducing not only skin aging but also skin cancer. Accordingly, a sunscreen cosmetic composition has become desired that exhibits an UV protective effect over a broad UV radiation region covering both the UV-B region and the UV-A region.
Especially with the recent skin-whitening boom, a sunscreen cosmetic composition much more excellent in the UV protective effect than before has become desired these days. Products additionally satisfying excellent feeling in use (non-sticky feeling, etc.) are more preferred. Further desired are low-viscosity ones that are lightly spreadable in application thereof.
As techniques close to the sunscreen cosmetic composition of the present invention, there may be exemplified ones described in the following Patent References 1 to 3.
Specifically, JP 2008-162930A (Patent Reference 1) describes an oil-in-water emulsion sunscreen cosmetic composition containing an oil-soluble UV absorbent, a water-soluble thickening agent (for example, acrylic water-soluble polymer, etc.), a water-soluble UV absorbent, and a specific hydrophilic nonionic surfactant, saying that the cosmetic composition is excellent in UV protective capability and stability, without detracting from the fresh-feeling thereof. The sunscreen cosmetic composition described in Patent Reference 1 secures the fresh-feeling thereof as being an oil-in-water emulsion, but on the contrary, has an undeniable defect in that its waterproofness is not sufficient as compared with a water-in-oil emulsion one.
JP 2007-217379A (Patent Reference 2) describes a water-in-oil emulsion sunscreen cosmetic composition which exhibits an excellent UV screening effect and an effect of preventing/inhibiting odor change with the passage of time and which is prepared by incorporating a neutralized salt of phenylbenzimidazolesulfonic acid into the aqueous phase (inner phase) of a water-in-oil emulsion system combined with an oily phase (outer phase) composed of octocrylene and hydrophobicated titanium dioxide and/or zinc oxide. However, Patent Reference 2 has neither description nor suggestion indicating that the water-in-oil emulsion sunscreen cosmetic composition could dramatically enhance the UV absorption capability thereof, as the boosting effect of the UV absorption capability thereof, owing to the combined use of the UV absorbent and the water-soluble polymer therein.
JP 2007-291094A (Patent Reference 3) describes a water-in-oil emulsion sunscreen cosmetic composition that comprises, as incorporated therein, surface-treated zinc oxide of which the surface is coated with (dimethicone/methicone) copolymer and an organic titanate and/or trialkoxyalkylsilane, saying that the cosmetic composition is excellent in spreadability and is excellent in the dispersion stability of the powder form thereof with the passage of time and in the UV protective effect. However, also in this Patent Reference 3, there is given neither description nor suggestion indicating that the water-in-oil emulsion sunscreen cosmetic composition could dramatically enhance the UV absorption capability thereof (as the protective boosting effect of the UV absorption capability thereof) owing to the combined use of the UV absorbent and the water-soluble polymer therein.